git-commit: introduce git-commit crate

Porting the commit code that is found in heartwood[0]. It represents a
parsed git commit object -- sans the id for the commit itself.

[0]: https://github.com/radicle-dev/heartwood/blob/16b22d9178fa6a1aaf2ecb01ec97e067399c5286/radicle/src/git.rs#L392-L575

Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>
X-Clacks-Overhead: GNU Terry Pratchett

git-commit/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "git-commit"
+version = "0.1.0"
+license = "MIT OR Apache-2.0"
+edition = "2021"
+authors = [
+  "Alexis Sellier <alexis@radicle.xyz>",
+  "Fintan Halpenny <fintan.halpenny@gmail.com>",
+]
+keywords = ["git", "radicle"]
+
+[dependencies]
+thiserror = "1"
+
+[dependencies.git2]
+version = "0.15.0"
+default-features = false
+features = ["vendored-libgit2"]